https://cache.nixos.org/ provides prebuilt binaries for Nixpkgs and NixOS. It is used automatically by the Nix package manager to speed up builds.
If you are having trouble, please reach out through one of thesupport channels with the results ofthis diagnostics script which will help us figure out where the issue lies.
For questions, or support, the support page from the NixOS website describes how to get in touch.